where did you see that on the Ford Performance site?
because this is what the Ford press release says about the appearance package.
For customers who want an even more sporty look, the Mustang Dark Horse Appearance Package includes specially bolstered RECARO® performance seats featuring exclusive Deep Indigo Blue bolsters trimmed with Bright Indigo Blue accent stitching, plus black Dinamica suede in the seat’s center for improved lateral grip. Deep Indigo Blue seatbelts and distinctive seat perforations with blue accents beneath the seating surface round out the blue theme.
Also here's an article about the appearance package.
Ford has released images and details of the Mustang Dark Horse's cabin with Indigo Blue accents. The automatic model receives anodized silver paddle shifters.
Those willing to spruce up the cabin furthermore can opt for the Mustang Dark Horse Appearance Package with body-hugging Recaro seats featuring Deep Indigo Blue bolsters trimmed with Bright Indigo Blue accent stitching. They're exclusive to this version and have black Dinamica suede centers. To sweeten the pot, Ford will also throw in Deep Indigo Blue seatbelts and seat perforations with blue accents.